The technical field relates to an electrical apparatus, and more particularly to an electrical apparatus, such as a surge arrester, that employs one or more housing segments.
Electrical transmission and distribution equipment is subject to voltages within a fairly narrow range under normal operating conditions. However, system disturbances, such as lightning strikes and switching surges, may produce momentary or extended voltage levels that greatly exceed the levels experienced by the equipment under normal operating conditions. These voltage variations often are referred to as over-voltage conditions.
If not protected from over-voltage conditions, critical and expensive equipment, such as transformers, switching devices, computer equipment, and electrical machinery, may be damaged or destroyed by over-voltage conditions and associated current surges. Accordingly, it is routine practice for system designers to use surge arresters to protect system components from dangerous over-voltage conditions.
A surge arrester is a protective device that is commonly connected in parallel with a comparatively expensive piece of electrical equipment so as to shunt or divert over-voltage-induced current surges safely around the equipment, and to thereby protect the equipment and its internal circuitry from damage. When exposed to an over-voltage condition, the surge arrester operates in a low impedance mode that provides a current path to electrical ground having a relatively low impedance. The surge arrester otherwise operates in a high impedance mode that provides a current path to ground having a relatively high impedance. The impedance of the current path is substantially lower than the impedance of the equipment being protected by the surge arrester when the surge arrester is operating in the low-impedance mode, and is otherwise substantially higher than the impedance of the protected equipment.
When the over-voltage condition has passed, the surge arrester returns to operation in the high impedance mode. This high impedance mode prevents normal current at the system frequency from flowing through the surge arrester to ground.
Conventional surge arresters typically include an elongated outer enclosure or sheath made of an electrically insulating material, such as porcelain, a pair of electrical terminals at opposite ends of the enclosure for connecting the arrester between a line-potential conductor and electrical ground, and an array of other electrical components that form a series electrical path between the terminals. These components typically include a stack of voltage-dependent, nonlinear resistive elements, referred to as varistors. A varistor is characterized by having a relatively high impedance when exposed to a normal system voltage level, and a much lower resistance when exposed to a larger voltage, such as is associated with over-voltage conditions. In addition to varistors, a surge arrester also may include one or more spark gap assemblies electrically connected in series or parallel with one or more of the varistors. Some arresters also include electrically conductive spacer elements coaxially aligned with the varistors and the spark gap assemblies.
In one general aspect, an electrical apparatus includes at least one housing segment and at least one MOV disk. The housing segment includes a sheath and defines a bore passing through the housing segment that has a first opening at one end of the housing segment and a second opening at the opposite end of the housing segment. The MOV disk is positioned within the bore. An adhesive is positioned in the bore between the MOV disk and the housing segment and is configured to circumferentially bond the MOV disk to the housing segment.
Embodiments of the electrical apparatus may include one or more of the following features. For example, the MOV disk may extend from the bore beyond the first opening and may further extend from the bore beyond the second opening. The electrical apparatus may further include a second MOV disk positioned within the bore and the adhesive may be positioned in the bore between the second MOV disk and the housing segment to bond the second MOV disk to the housing segment. The first MOV disk may or may not be in contact with the second MOV disk.
The electrical apparatus also may further include a second housing segment that encloses a portion of the first MOV disk. The first housing segment may or may not be in contact with the second housing segment. The electrical apparatus may further include a coating, such as a hydrophobic coating, applied to the housing segments.
The housing segment may further include an annular projection extending from a wall of the housing segment into the bore and the annular projection may include a passage through the annular projection. The MOV disk may be adjacent to a first side of the annular projection. The housing may include a second MOV disk that is positioned adjacent to a second, opposite side of the annular projection and an adhesive may be positioned in the bore between the second MOV disk and the housing segment. The second MOV disk may be arranged to be out of physical contact with the first MOV disk, but in electrical contact with the first MOV disk through an electrically conductive element. The housing may further include a second housing segment enclosing at least a portion of the second MOV disk and an adhesive positioned in the bore between the second MOV disk and the second housing segment. The second housing segment may or may not be in contact with the first housing segment.
In another general aspect, constructing an electrical apparatus includes providing a housing segment, positioning a MOV disk within the housing segment, and using an adhesive to bond the MOV disk to the housing segment. The housing segment includes a sheath and defines a bore passing through the housing segment and having a first opening at one end of the housing segment and a second opening at the opposite end of the housing segment. The MOV disk has a first end and a second end within the bore such that the second end extends from the second opening in the bore. The adhesive is placed in the bore between the MOV disk and the housing segment.
Embodiments may include one or more of the features described above, as well as the following features. For example, constructing the electrical apparatus may further include providing a second housing segment, positioning a second MOV disk, placing an adhesive between the MOV disk and the housing segment; inserting the first MOV disk into the second housing segment, and placing an adhesive between the first MOV disk and the second housing segment. The second housing segment includes a sheath and defines a bore having a first opening at one end of the second housing segment and a second opening at the opposite end of the second housing segment. The second MOV disk has a first end and a second end within the bore of the second housing segment such that the second end extends from the second opening in the bore. Inserting the first MOV disk includes inserting the second end of the first MOV disk into the first end of the second housing segment. The adhesive placed between the first MOV disk and the second housing segment is placed in the bore between the first MOV disk and the second housing segment.
Constructing the electrical apparatus may further include providing additional housing segments and MOV disks, mounting them to one or both of the first MOV disk and the second housing segment, and bonding the MOV disks to the housing segments using an adhesive. A coating, such as a hydrophobic coating, may be applied to the housing.
The first housing segment may be in contact with the second housing segment but may not be in contact with the second housing segment such that an exposed portion of each MOV disk is not enclosed by the housing segments and the coating is applied to the exposed portion of each MOV disk.
Each housing segment may include an annular projection extending from a wall of the bore into the bore and the first end of the first MOV disk may be adjacent to the annular projection and the second end of the second MOV disk may be adjacent to an opposite side of the annular projection. The annular projection may include a passage through the annular projection and an electrically conductive element may be placed in the passage whereby the first MOV disk is in electrical contact with the second MOV disk.
The use of circumferentially bonded housing segments provides considerable advantages. For example, the circumferential bonds between the housing segments and the MOV disks are strong enough to prevent the surge arrester from breaking when subjected to horizontal forces. The methods of circumferentially bonding the housing segments to the MOV disks allows the manufacturer to produce and store subassemblies that can be assembled easily into an electrical apparatus when an order is received. The exposed disk surge arresters provide the advantage of reducing the amount of material used to form the weather shed, which reduces costs.